Effects of copper or germanium additions on the stabilized formation of α-phase formamidinium lead triiodide perovskites

Formamidinium lead triiodide (FAPbI₃) is one of the candidate materials for stable perovskite solar cells. There exists an optically active cubic α-FAPbI₃ phase, an optically inactive hexagonal δ-FAPbI₃ phase, and a one-dimensional phase. Since the δ-phase is thermodynamically stable, the structural phase transition from the α-phase to the δ-phase causes a serious problem on the photovoltaic efficiencies. The aim of this study is to investigate the effects of copper (Cu) or germanium (Ge) additions on the formation of FAPbI₃. Cu or Ge were added in the present study, as a method for stabilizing the α-phase, to suppress formation of the δ-phase and one-dimensional phase. When Cu was added at the lead site, diffraction peaks of the α-phase increased. Ge addition also increased the diffraction intensity of the α-phase and decreased the diffraction intensity of PbI₂. The possibility of stabilization of FAPbI₃ by Cu introduction at the FA site was also demonstrated. The first-principles band calculation on the Cu-doped FAPbI₃ at the FA site indicated that the total energy value of the crystal decreased. From the calculated partial density of states, the valence band and conduction band are dominated by I-p orbitals and Pb-p orbitals, respectively, and the energy level of the Cu-d orbital is formed at a position slightly lower than the valence band maximum. The effectiveness of Cu introduction in stabilizing the formation of α-FAPbI₃ was also demonstrated in the synthesized FAPbI₃ crystal.